1. Field of the Invention
This invention relates to the field of measuring nuclear magnetic resonance (NMR) properties of an earth formation.
2. Description of the Related Art
Geologic formations below the surface of the earth may contain reservoirs of oil and gas. Measuring properties of the geologic formations provides information that can locate the reservoirs of oil and gas. Typically, the oil and gas are retrieved by drilling boreholes into the subsurface of the earth. The boreholes also provide access to take measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the boreholes. A “logging instrument” (also referred to as a “logging tool”) is lowered on the end of a wireline into the borehole. The logging instrument sends data via the wireline to the surface for recording. One type of measurement involves measuring nuclear magnetic resonance (NMR) properties from the geologic formations.
A new generation of NMR logging instruments based on using permanent magnets was introduced into practice about a decade ago (see for example U.S. Pat. No. 4,717,878 issued to Taicher et al., U.S. Pat. No. 5,055,787 issued to Kleinberg et al., and U.S. Pat. No. 6,452,388 issued to Reiderman, et al.). The new generation of NMR logging instruments has demonstrated obvious advantages over the old technologies (earth's magnetic field NMR logging) such as high resolution in acquiring NMR relaxation spectra, capability of diffusion measurements, as well as a defined volume of investigation with no effect of protons in the bore fluid on acquired NMR data.
It was recognized though that a high rate of decreasing static and radio frequency magnetic fields with distance from the NMR sensor results in a relatively small and shallow region of investigation. Consequently, an acceptable signal-to-noise ratio (SNR) can be achieved only at depths of investigation in a range of about one to four inches (typically corresponds to an invasion zone). Even in this range, the SNR is often not sufficient to allow for a desired logging speed.
Drawbacks of new NMR tools stimulated interest toward alternative designs including ones based on a modified early concept of an earth's magnetic field NMR logging tool. A typical earth's magnetic field NMR tool creates a static magnetic field to polarize nuclei of elements present in an earth formation. The static magnetic field is turned off to allow free or driven precession of the polarized nuclei about the earth's magnetic field. Signals generated by the free precession of nuclear spin magnetic moments are detected and correlated to elements in the earth formation. U.S. Pat. No. 3,538,429 issued to John H. Baker and U.S. Pat. No. 4,035,718 issued to Richard N. Chandler give examples of a technique that uses an abrupt (non-adiabatic) turning off of the static magnetic field and detecting signals generated by free precession of nuclear spin magnetic moments. U.S. Pat. No. 6,366,086 issued to Pabitra N. Sen addresses an issue of an undesirable signal acquired from borehole fluid. Drawbacks of techniques of this type are a poorly defined excitation region as well as a relatively low SNR due to power-consumption-limited intensity of the static magnetic field used to polarize the nuclei.
None of the methods and apparatus of prior art have been able to facilitate deep (on an order of one meter) NMR measurements. Facilitating the deep NMR measurements typically requires (1) generation of a large magnetic dipole as a source of the static magnetic field with a capability of switching the static magnetic field “off” in microseconds and (2) suppression of strong NMR signals emanating from shallow regions close to the borehole.
What are needed are a method and apparatus to produce the static magnetic field with the switching capability and to substantially eliminate the strong NMR signals emanating from shallow regions close to the borehole.